The present invention relates to optical communication systems and, in particular, to an optical communication system incorporating one or more automatic chromatic dispersion compensation modules for optimizing system performance.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, optical fiber system comprises a source of information-carrying optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the optical signals and demodulating the information they carry. Optical amplifiers are typically located along the line at regular intervals, and add/drop nodes are disposed at suitable locations for adding and dropping signal channels.
Optical communication systems are usually based on high purity silica optical fiber as the transmission medium. Conventional systems are typically designed to transmit optical signals in a wavelength range where longer wavelength components are subject to slightly longer propagation time delay than shorter wavelengths. This chromatic dispersion did not deteriorate the information content of the optical signals because early systems used a single channel at a wavelength where dispersion is low.
As it has become desirable to utilize many channels over a wider range of optical wavelengths (WDM systems), group velocity dispersion has required more precise compensation. WDM systems are becoming increasingly important for their ability to transmit vast amounts of information and for their ability to incorporate network functions such as add/drop and cross connecting. But as the number of channels increases in WDM systems, dispersion compensation becomes increasingly important.
Dispersion gives rise to undesirable pulse distortion, which can limit bandwidth and/or transmission distances. Dispersion compensation is thus critical to the performance and ultimate commercial success of communication systems and particularly of those that operate at 10 Gbit/s per wavelength channel and higher. Typically, dispersion compensation is accomplished using especially designed optical fibers possessing specified dispersions or with chirped fiber gratings, both of which work by providing a fixed amount of dispersion of an equal sign and opposite magnitude to that of a given fiber span. Both of these technologies have been demonstrated and are being incorporated into lightwave systems.
The performance of high speed WDM lightwave networks will depend critically on the details of the system design and particularly on the level of in-line dispersion and dispersion slope compensation as well as nonlinear effects occurring in the dispersion compensating fiber (DCF). In such systems small variations in optical power, due for example to imperfect gain flattening of optical amplifiers, can result in additional nonlinear phase shifts that can modify the optimal dispersion map of the system. This problem is exacerbated by a reduced dispersion budget associated with imperfect dispersion slope compensation over a wide bandwidth of operation. For example, in a typical system operating with approximately 40 nm of bandwidth, and with an uncompensated dispersion slope of 0.05 ps/nm2 km, the accumulated divergence in the dispersion (assuming approximately 60% compensation in DCF) is approximately 1.2 ps/nm km. The corresponding dispersion budget is typically taken as twice this value, giving 2.4 ps/nm km. It follows that the maximum transmission distance (L) that can be achieved before incurring a significant penalty is given by
L less than 104,000/(B2|D|)(Gb/s)2ps/nmxe2x80x83xe2x80x83(1)
where B is the channel rate and D the dispersion of the fiber, is 32 km for a 40 Gbit/s system and 512 km for a 10 Gbit/s system. Therefore, in systems that operate at 10 Gbit/s, 40 Gbit/s or higher, the combination of dynamic fluctuations in the optical power and the reduced dispersion budget associated with the dispersion slope will impose significant challenges in designing networks.
Because of these variations with wavelength and level fluctuations, it is very difficult, with fixed or static dispersion compensating devices, such as DCF or conventional chirped Bragg gratings, to manage effectively the unavoidable dispersion and nonlinearities in a high speed WDM lightwave network. Optical networks typically have slow variations in the optimal path dispersion that arise whenever there is a change in the total optical power and/or power distribution in the network.
FIG. 1 shows an exemplary conventional optical fiber transmission system. The system comprises a transmitter 10, a transmission fiber path 11 and a receiver 12. The system may also include one or more static dispersion compensation modules 14. The transmission path typically comprises conventional optical transmission fiber, and optionally comprises one or more optical amplifiers 13 (typically erbium amplifiers). At the transmitter output the pulse shape is sharp and well defined. After passage through, typically hundreds of kilometers of optical fiber 11, including a multiplicity of optical amplifiers 13, the pulse has broadened, and is considerably distorted. After transmission through a (static) dispersion compensation module 14 the pulse is reshaped and is mostly restored into its original shape.
In realistic optical systems, significant nonlinearities occur in the transmission fiber and in other components of the network. These nonlinearities prevent complete dispersion compensation with a static module. With a fixed dispersion compensating element, optimization is difficult because nonlinearities give rise to additional phase shift and pulse distortion that are dependent on optical power and therefore change with time as the traffic on the network changes or the gain of the erbium-doped amplifiers change. It can therefore be very difficult to ensure complete restoration of the pulse at all points and at times in any realistic high-bit optical network. This results in pulse distortion that is not compensated for, can accumulate and can significantly degrade system performance.
In addition, in optical systems comprising more than one channel (Wavelength Division Multiplexed systems) the incurred pulse distortion can be channel dependent and, as before, can vary with time. In such a system, one requires the ability to add or to drop specific channels at selected nodes in the network. This adding and dropping can be accomplished, for example, with fiber gratings that reflect the desired channel and transmit all other channels. In performing add/dropping, the average power in the fiber can fluctuate. Gain flatteners are capable of accommodating for such changes, but small fluctuations in the optical power are unavoidable. As a consequence of this power fluctuation, the pulse can undergo more pulse distortion in the transmission fiber and thus can require a different and time-varying amount of dispersion compensation.
Several approaches to eliminate the effects of unwanted dispersion have been proposed: (a) optical pulse regeneration (b) optical phase conjunction (OPC); (c) optical solitons. All of these techniques are promising and have been demonstrated in laboratory environments, but they have important limitations for application to realistic systems such as those being deployed now. For example, OPC (also referred to as mid-span spectral inversion) requires that the dispersion compensation must be performed at the mid-point of the link. This requirement is often not possible to satisfy with many optical network designs. Soliton systems are very attractive, but they require precise management of the dispersion map of the system and suffer other disadvantages.
The use of dynamic dispersion elements in communication systems has been proposed to compensate for a change in the fiber dispersion resulting from a network reconfiguration (see J. X. Cai et al., Proceedings of Optical Fiber Conference, 1998, page 365 (1998)). The reference proposes the use of a dispersion monitor and a dynamic dispersion element to compensate for transmission pathlength changes. The monitor measures the total chromatic dispersion between the transmitter and the receiver. However, this approach measures only the dispersion and cannot directly measure the integrity of transmitted data. This is a critical flaw because the data is also distorted during transmission through nonlinear effects, where accumulated phase shifts not measured by a dispersion monitor, vary the dispersion compensation required to achieve optimum performance. Thus there is a need for an improved automatic dispersion compensation module to optimize system performance.
In accordance with the invention, an optical communication system is provided with one or more automatic dispersion compensation modules. Each module has an adjustable dispersion element, a data integrity monitor and a feedback network whereby the monitor adjusts the dispersion element to optimize system performance. In a preferred embodiment the dispersion compensating modules comprise chirped fiber Bragg gratings in which the chirp is induced in the grating by passing a current along distributed thin film heaters deposited along the length of the fiber. The magnitude of the applied current determines the dispersion of the grating. A data integrity monitor is configured to sense the integrity of transmitted data and to provide electrical feedback for controlling the current applied to the grating.